Traffic control systems generally include devices for monitoring a roadway for determining vehicle traffic flow on particular routes, along with devices for controlling traffic signals in response to the determined traffic flow. Ideally, the systems determine the presence of each moving vehicle on the monitored roadway, the speed of that vehicle, and the presence of a stopped vehicle. From that information, together with other desired information such as road conditions, traffic on intersecting roadways and other routes, and other measurable parameters, the system can control traffic signals to optimize traffic flow and to determine traffic patterns and needs.
A common device for installation in a roadway to determine traffic flow is a "loop". A loop is a loop of electrically conductive wire measuring roughly six feet by six feet which is buried in the roadway and connected to a resonant circuit. The wire carries an alternating current whose frequency is related to the inductance. The presence of a ferromagnetic material such as a vehicle causes the loop inductance to change which in turn causes the frequency of the resonant circuit to change in a detectable and predictable way.
Loop systems have several drawbacks. One drawback is that the resolution of the detector is limited by the relatively large size of the loop of wire. Another drawback is the difficulty and expense of installing the loop of wire. Installation of the loop of wire generally entails saw-cutting a slot into the roadway in a rectangular pattern, carefully placing the loop wire into the slot, and then filling and sealing the slot with epoxy, rubber or other suitable (and typically inconvenient) material. The sealing step is both important and difficult because the sealing is to fix the loop in place to prevent any horizontal or vertical movement of the buried wires so there are not spurious signals due to wire movement. It is also to prevent water from reaching the wires, which could change the loop inductance. Another drawback to loop systems is that their performance is affected by the quality of the roadway. Irregularities or fractures in a paved road may produce false signals in a loop system, and a loop system is generally not feasible at all on a gravel or other unpaved surface. Another drawback is that the saw cuts in the pavement that are required for the installation of the loop wire immediately damages the pavement and leads to further long-term damage by weakening the roadway and creating a point for further wear and water accumulation from frost, erosion, snowplows, chemicals and traffic.
Due to these difficulties in installing the wire of a loop system and the weakening of the pavement associated with the installation, the life of the loop wire is much shorter than desired. When the wire fails it must be replaced, again at considerable time and expense, and the replacement requires closing at least one traffic lane. The replacement wire will likely be no more reliable or long-lived than the wire it replaced.
Another approach to detecting vehicles is by the use of magnetic probes, or "magnetometers", such as those described in U.S. Pat. No. 4,449,115 by Koerner and U.S. Pat. No. 3,984,764 by Koerner, the contents of both of which are hereby incorporated by reference. A magnetometer is a device to detect irregularities in the flux of the earth's magnetic field caused by nearby ferromagnetic materials. The earth's magnetic field normally produces fairly uniformly spaced flux lines within a small surveillance area. However, a ferromagnetic material has a lower path of reluctance for the flux lines, which causes the flux lines to warp so that more lines and a higher density of lines pass through the material as it occupies a given space than pass through the same space not occupied by the material. The electrical windings of the magnetometer detect and measure these irregularities in magnetic flux density.
Magnetometers are preferred over loop systems because magnetometers are small, with a length of less than two feet and a diameter of no more than a few inches. Therefore, the magnetometer can be installed in a conduit that is permanently built under the roadway. The conduit can be accessed by a manhole cover or other suitable means, so that the installation and replacement of the magnetometer is a fairly simple matter that requires no roadwork.
A disadvantage to the use of both loop systems and magnetometers is that they readily detect passing vehicles, but they do not readily compute the speed of that vehicle. In order to compute the speed of a vehicle, it is generally necessary to space a pair of loops along the roadway to measure the time between detection of the vehicle at the first loop and detection of the vehicle at the second loop, and then compute the vehicle speed by dividing the spacing distance by the elapsed time. Of course, having two loops rather than just one increases the trouble, time and expense of installing and maintaining the system.
Several systems have attempted to obtain information about the speed of a vehicle from a single detector. For example, U.S. Pat. No. 4,379,280 by Eshraghian describes a vehicle detector wherein the envelope of a received continuous wave signal is analyzed to predict the speed of a passing vehicle. In U.S. Pat. No. 4,862,163 by Sobut, there is described a speed detector employing a pressure transducer which generates a signal in response to the weight of a passing vehicle. A long signal indicates a slow-moving vehicle while a short signal indicates a faster-moving vehicle. Other art in the field includes U.S. Pat. No. 4,862,162 by Duley (which includes a calibration procedure to account for environmental changes); U.S. Pat. No. 4,430,636 by Bruce; U.S. Pat. No. 4,968,979 by Mizumo; U.S. Pat. No. 4,916,621 by Bean; U.S. Pat. No. 4,368,428 by Djikman; U.S. Pat. No. 4,358,749 by Clark; U.S. Pat. No. 4,075,553 by Bouverot; U.S. Pat. No. 4,232,285 by Narbaits-Jaureguy; U.S. Pat. No. 3,944,912 by Angel; and U.S. Pat. No. 5,008,666 by Gebert. In none of this art is there a system for determining the speed of a passing vehicle by analyzing the profile of a digitized signal generated by a magnetic probe and for determining the presence of a stopped vehicle in the manner of the present invention.